Parking meters permit vehicles to be parked on streets for an allowable time determined by the number and denominations of coins which are placed in the parking meter. A clock mechanism in the parking meter runs down the allowable time until it reaches zero, and an overtime parking indication appears.
It has been long recognized that if the parking meter were able to detect the presence or absence of the vehicle, either by mechanical means or wireless means, in the corresponding parking space, then among other things, the parking meter could be reset, thereby requiring the next patron to insert the appropriate amount of payment for his/her parking time. U.S. Pat. No. 3,015,208 (Armer); U.S. Pat. No. 3,018,615 (Minton et al.); U.S. Pat. No. 3,034,287 (Odom et al.); U.S. Pat. No.3,054,251 (Handley et al.); U.S. Pat. No. 3,064,416 (Armer); U.S. Pat. No. 3,535,870 (Mitchell); U.S. Pat. No. 3,999,372 (Welch); U.S. Pat. No. 4,043,117 (Maresca et al.); U.S. Pat. No. 4,183,205 (Kaiser); U.S. Pat. No. 4,823,928 (Speas); U.S. Pat. No. 4,825,425 (Turner); U.S. Pat. No. 4,908,617 (Fuller); U.S. Pat. No. 4,967,895 (Speas); U.S. Pat. No. 5,442,348 (Mushell); U.S. Pat. No. RE29,511 (Rubenstein).
Thus, the objective of any vehicle detection portion of the electronic parking meter is to, as reliably as possible and as inexpensively as possible, detect when there is and is not a vehicle in the corresponding parking space. In fact, experience has shown that unless vehicle detection is extremely reliable (99%+ in correctly identifying the presence/absence of a vehicle), the customer, i.e., cities and townships, will not invest in vehicle detecting parking meters. However, all of the above references suffer from one of many different problems and actually achieving this objective remains elusive. The reasons for not being able to implement such a working vehicle detector include: the uncertainty of the parking meter location and of the parking meter/space environment, vehicles that are parked too far back in the parking space, the smoothness of the surfaces of different vehicles, the "fast parker", the inadvertent or intentional presence of a person in front of the meter and tampering with the meter including the vandalizing of the sensor itself. Furthermore, the vehicle-detecting parking meter must be able to provide a reliable vehicle-detection scheme that uses low power since the parking meter is a stand-alone device that does not have the luxury of using utility power.
In particular, the environment of the meter/space presents obstacles that must be recognized and compensated for, or distinguished, by the vehicle detector. For example, the road may be very steeply-crowned and an ultrasonic-based vehicle detector will receive reflections from the crowned road, and may erroneously conclude that a vehicle is in the corresponding parking space when there truly is no vehicle there. Another example, is that if trash bins, light posts, trees, sign posts, etc. are closely-adjacent the parking meter, almost any wireless vehicle detection scheme will be subjected to sufficient interferences from these, thereby causing the detector to make erroneous conclusions about the presence/absence of a vehicle in the parking space.
Even the sensor used to implement the vehicle detection suffers from its own respective drawbacks. For example, the use of RADAR (radio detection and ranging) suffers from such things as possible interferences from other RADAR-vehicle-detecting units, frequency band licensing concerns as well as cost. The use of optical sensors in vehicle detection (e.g., U.S. Pat. No. 4,043,117 (Maresca)) suffer from receiving reflections that may vary from strong reflections (reflected off of vehicle glass) versus weak reflections (reflected off the body of a very dark-colored vehicle), which are hard to detect. Videocamera/processing when used for vehicle detection (e.g., U.S. Pat. No. 5,777,951 (Mitschele et al.)) is not only very expensive but in those cases where the video camera is positioned to capture the front-end vehicle license plate, in those states where front-end vehicle license plates are not required, identification of the vehicle is thwarted. Thus, at present, use of ultrasonic sensors remains the most cost-effective means of detecting vehicles.
Prior art vehicle detecting parking meters utilizing a single ultrasonic sensor, such as those disclosed in U.S. Pat. Nos. 5,407,049 (Jacobs), 5,454,461 (Jacobs), U.S. Pat. No. 5,570,771 (Jacobs), U.S. Pat. No. 5,642,119 (Jacobs) and U.S. Pat. No. 5,852,411 (Jacobs et al.), which are assigned to the same assignee as the present invention, namely Intelligent Devices, Inc., operate where the ultrasonic sensor is energized with a pulse for emanating an interrogating signal towards the parking space and then the sensor waits to receive reflections. In particular, the reflections are examined to determine if they exceed a certain fixed threshold and, if so, the time measured between when the interrogating signal was sent until when the reflection was received is used to calculate a distance.
However, some of the problems with such a method are the following: certain vehicles disperse the interrogating signal, rather than returning a strong reflection; another problem is that to compensate for adjacent obstacles, e.g., crowned-street, tree, sign post, etc., the sensitivity of the sensor has to be reduced by raising the threshold but in doing so, even more vehicles are not properly detected; the reflected signals, or echos, are inherently unstable, i.e., the movement of air and even very minute physical movements in the environment make these signals unstable. Furthermore, some echos cancel other echos and exhibit multi-path problems, thus making the echos unstable.
Even where multiple ultrasonic sensors are used to detect vehicles, e.g., U.S. Pat. No. 3,042,303 (Kendall et al.); U.S. Pat. No. 3,046,519 (Polster); U.S. Pat. No. 3,046,520 (Polster); U.S. Pat. No. 3,105,953 (Polster); U.S. Pat. No. 5,263,006 (Hermesmeyer); U.S. Pat. No. 4,845,682 (Boozer et al.), or other objects U.S. Pat. No. 5,761,155 (Eccardt et al.), the design is that at least one sensor acts as an ultrasonic transmitter and the remaining sensors act as the ultrasonic receivers. As a result, there is no teaching or suggestion that each sensor act as both a transmitter/receiver for a signal that monitors a particular portion of the parking space. Furthermore, low power operation of these system is not a concern.
Another problem that is encountered with such vehicle detection systems is a "fast-parker" scenario, i.e., a vehicle pulling into a parking space that has just been emptied but before the vehicle detector has determined that the first vehicle has departed.
With regard to low power electronic parking meters, British Publication No. 2077475 discloses a low power electronic parking meter that operates using solar cells. Furthermore, since the sophisticated electronic parking meters which use microprocessors, electronic displays and IR/ultrasonic transducers consume too much power to operate by non-rechargeable batteries alone, U.S. Pat. No. 4,967,895 (Speas) discloses the use of solar power cells which charge capacitors or rechargeable batteries. However, various problems exist with the use of solar power sources including the use of parking meters in shady areas, or the use of parking meters during periods in which there is very little sunlight. This causes the rechargeable batteries to run down, and they require frequent replacement. Or, in the case of the use of capacitors, the lack of power causes the meter to become inoperative.
Therefore, there remains a need a system and method for providing any electronic parking meter with the ability to detect the presence or the absence of a vehicle in any existing parking meter space, independent of the surrounding environment, as reliably as possible and as inexpensively as possible while using a minimum of power.